The present invention relates to oximetry sensors, and in particular to pulse oximetry sensors which attach across a patient's appendage.
Pulse oximeter sensors are often attached to a digit, or ear. These sites on a patient provide an adequate level of blood perfusion for measuring the oxygenation of the blood hemoglobin. In addition, the distance across these appendages is sufficiently short to allow the detection of transmitted red or infrared light.
One type of sensor is a clothespin-type clip which attaches across the earlobe, with the emitter and detector opposite each other. Such conventional sensors sometime suffer from poor sensitivity and poor calibration or accuracy.
This type of sensor often applies pressure which exsanguinates the tissue and alters the blood present leading to accuracy errors.
One type of oximeter sensor will add a diffusing optic to diffuse the light emitted from the light-emitting diodes (LEDs) to cause it to pass through more tissue, and thus more blood. An example of a pulse oximeter sensor using such a diffusing element is shown in U.S. Pat. No. 4,407,290.
One technique for limiting the exsanguination effect is to separate the light emitters and detector from the portion of the sensor which holds it to the appendage and applies the pressure. Examples of sensors where the light emitters and detector avoid the point of pressure are set forth in U.S. Pat. Nos. 5,413,101 and 5,551,422.
Another type of clip-on sensor is marketed by Nonin Medical, Inc. for attaching to an ear. Instead of using a transmission sensor where light is transmitted from an emitter on one side of the ear through the ear to a detector on the other side, a reflectance sensor is used with both the emitter and detector on the same side of the ear. The Nonin medical sensor has spacing between the emitter and the detector of approximately 4 mm, which is similar to the thickness of a typical earlobe. On the opposite side of the ear a reflective surface is provided to reflect the light from the emitter back to the detector.
The typical distance of a standard, band-aid-type reflectance sensor which can attach to the forehead or other part of the body is 6-10 mm. Traditionally, a spacing of this magnitude was felt to be appropriate to ensure that a measurable amount of light could be detected with sufficient pulsatile signal components.